from .xyz import FunctionA also imports FunctionB and FunctionC - confused why
A blender addon is a python module. Using bpy.utils.register_module(__name__) will register every class within the module, which includes classes in all .py files within a module made of multiple files.

Don't confuse importing with blender's registering process. By importing you are bringing the class into the current namespace, which could be a methods namespace, but blender's register causes a suitable class to be registered as an operator under bpy.ops making it available to display in a menu using the layouts operator() method.